when the traffic volume, i.e. the number of call attempts rises above the maximum capacity of a switching center, then the offered load (rate of call attempts) can no longer be handled with the necessary speed and an overload of the switching center occurs.
In order to prevent overload in a switching center, some of the call attempts that come from outside the switching center (incoming call attempts) must be rejected. The rejection of the call attempts should thereby be implemented gradually according to the existing overload, i.e., call attempts are increasingly rejected as the overload increases in the switching center. Every processing request to the controller of the switching center that arrives from outside the switching center is to be considered an incoming call attempt.
According to the specification EWSD, Overload handling, A30808-X2717-X-2-7618 of Siemens AG, it is known to control overload prevention using seven overload levels, whereby overload level 0 denotes that all call attempts are accepted and overload level 6 denotes that only 0% through 5% (residual acceptancy) of the incoming call attempts are accepted.
An equal distribution is desired therebetween, i.e. when the overload level rises by one level, approximately 16% fewer call attempts are accepted.
Every port which is equipped with a subscriber line or a trunk has one of seven possible priority levels allocated to it. Priority level 0 is thereby equivalent to the lowest priority and priority level 6 is equivalent to the highest priority.
A call attempt is rejected when the priority level of the appertaining port is lower than the overload level of the switching center.
A distinction is made in the case of the subscriber lines between prioritized and non-prioritized subscriber lines. The non-prioritized subscriber lines (normal subscriber lines) are uniformly divided between the four priority levels 0 through 3. The division occurs automatically on the basis of a linkage of the port address with the time of day. These ports cyclically change their affiliation to priority level approximately every thirty seconds according, for example, to the following sequence: EQU 0.fwdarw.1.fwdarw.2.fwdarw.3
thus each of these normal ports, is in priority levels 0, 1, 2 and 3 with equal frequency over a time span that is significantly longer than thirty seconds.
Fixed-priority ports are identified as such by what is referred to as a MML command and have priority level 5 allocated to them. The MML command represents a command of the operating program of the switching center and is input by the operating personnel of the communication network. Call attempts from subscribers having a fixed-priority port are only rejected given an extremely high overload (overload level 6).
The priority levels of the ports of trunks are fixed, i.e. are chronologically invariable, and are allocated thereto bundle-by-bundle by the MML command, i.e., the ports of a specific bundle of trunks all have the same priority.
The priority levels of the bundles are thereby assigned according to network parameters. In local offices and local transit switching centers, the priority levels 3 through 6 are used for trunk bundles. The trunk bundles are distributed over priority levels 0 through 6 for exclusive long-distance and transit switching centers.
Unfortunately, the type of assigning for the priority levels of trunk bundles by the operating personnel of the switching center operator, is often unprofessionally implemented, or sometimes not even at all. This has a correspondingly negative effect on the behavior of the switching center in case of an overload.